1. Field of the Invention
The present invention generally relates to a frequency synthesizer used in a wireless mobile appliance, and more specifically, to a frequency synthesizer capable of manufacturing a VCO in an IC form, resulting in low cost.
2. Description of the Related Art
In wireless mobile appliances such as portable telephones, frequency synthesizers are used to generate arbitrary local oscillation frequencies from a reference signal.
In general, as indicated in FIG. 5, a frequency synthesizer used in a wireless mobile appliance such as a portable telephone is equipped with a VCO 1, a prescaler 2, a counter 3, a reference frequency divider 5, a phase comparator 6, a charge pump 7, and a loop filter 8. Both the prescaler 2 and the counter 3 constitute a pulse-swallow type variable frequency divider. The VCO 1 oscillates a signal having a frequency in response to a voltage which is applied to a frequency control voltage terminal of the frequency synthesizer. The prescaler 2 divides a frequency of an output signal (will be referred to as an xe2x80x9cfVCOxe2x80x9d hereinafter) of the VCO 1. The counter 3 counts the output signal of the prescaler 2. The reference frequency divider 5 divides a frequency of an output signal (will be referred to as an xe2x80x9cfOSCxe2x80x9d hereinafter) of a reference signal source 4. The phase comparator 6 compares a phase of an output signal (will be referred to as an xe2x80x9cfdivxe2x80x9d hereinafter) of the counter 3 with a phase of an output signal (will be referred to as an xe2x80x9cfrefxe2x80x9d hereinafter) of the reference frequency divider 5 to thereby output a phase difference. The charge pump 7 converts the output signal of the phase comparator 6 into either a voltage or a current. The loop filter 8 averages the output signal of the charge pump 7.
FIG. 6 is a circuit diagram for representing a basic operation of the VCO disclosed in Japanese Laid-open Patent Application No. Hei-10-261918. This circuit is provided with the capacitor xe2x80x9cCOxe2x80x9d, the negative-characteristic resistive portion xe2x80x9cxe2x88x92Rxe2x80x9d, and the inductor xe2x80x9cLxe2x80x9d, which are connected in parallel to each other. This circuit is further equipped with the capacitor xe2x80x9cC1xe2x80x9d and the variable capacitance diode xe2x80x9cCvxe2x80x9d, which are connected in the cascade connection. This cascade connection between the capacitor C1 and the variable capacitance diode xe2x80x9cCvxe2x80x9d is connected in parallel to the capacitor CO.
Next, operations of the circuit shown in FIG. 6 will now be explained. The parallel connection portion constructed of the negative-characteristic resistive portion xe2x80x9cxe2x88x92Rxe2x80x9d, the capacitor xe2x80x9cCOxe2x80x9d, and the inductor xe2x80x9cLxe2x80x9d corresponds to a parallel resonant circuit containing an active element which generates electric power of a transistor and the like, to which the power supply voltage is applied. The negative-characteristic resistive portion xe2x80x9cxe2x88x92Rxe2x80x9d is different from the normal resistor, considering such an implication that electric power is generated.
The oscillation frequency of this VCO is expressed by the following formula [1]:
fVCO=xc2xdxcfx80L[C0+C1xc2x7Cv/(C1+Cv)]xe2x80x83xe2x80x83[1]
In the case that this voltage-controlled oscillator is employed in the frequency synthesizer of FIG. 5, the control voltage is applied to the variable capacitance diode xe2x80x9cCvxe2x80x9d, so that the capacitance value of the variable capacitance diode xe2x80x9cCvxe2x80x9d is varied. As a result, the oscillation frequency xe2x80x9cfVCOxe2x80x9d is varied.
In such a frequency synthesizer, when the count value of the counter 3 is changed, the frequency xe2x80x9cfdivxe2x80x9d is changed in response to this change in the count values. As a result, the phase comparator 6 outputs the phase error. Based upon this phase error, voltage of the frequency control voltage terminal of the VCO 1 is changed via the charge pump 7 and the loop filter 8 so as to vary the frequency xe2x80x9cfVCO.xe2x80x9d As previously explained, the frequency synthesizer constitutes the negative feedback loop, and finally locks the phase when the phase of the frequency xe2x80x9cfrefxe2x80x9d is made coincident with the phase of the frequency xe2x80x9cfdivxe2x80x9d, so that the output frequency of the VCO 1 can be made stable.
Generally speaking, in such a frequency synthesizer, a VCO is constituted by a module component. This module component will constitute one of major factors which may impede a compactness of a portable telephone and the like. Thus, such a VCO is desirably manufactured in the form of an IC component. However, in the case that such a VCO is manufactured in the IC component form, an oscillation frequency of this VCO would be largely varied due to manufacturing fluctuations occurred in electronic components for constituting this VCO. As a result, there is such a problem that the frequency synthesizer could not be phase-locked at the desirable frequency.
To solve this problem, there is one method capable of increasing a control sensitivity (namely, change width of oscillation frequency per 1V: unit being [Hz/V]). However, if the control sensitivity is increased, then there is another problem. That is, when the control sensitivity is increased, the frequency synthesizer may be easily and adversely influenced by outer disturbance noise, so that the CPU characteristic would be deteriorated.
Also, there is a further method for solving the above-explained problem. That is, in this method, a plurality of fixed capacitances are connected in parallel to a parallel resonant circuit of a VCO, and when this VCO is manufactured in the IC component form, these fixed capacitances are trimmed by laser and the like so as to set the oscillation frequency of the VCO. However, since the IC is separately adjusted, the manufacturing cost would be increased, resulting in another problem.
The present invention has been made to solve the above-described conventional problems, and therefore, has an object to provide a frequency synthesizer having a better C/N characteristic, by which a VCO having a wide output frequency range can be manufactured in an IC form, while realizing low cost.
To achieve the above-explained object, a frequency synthesizer, according to the present invention, is featured, by that in a frequency synthesizer having a switching means for switching a capacitor, or an inductor; and equipped with: a voltage-controlled oscillator (will be referred to as a xe2x80x9cVCOxe2x80x9d hereinafter) for oscillating a signal having a frequency in response to a voltage applied to a control voltage terminal; a first frequency divider for outputting a signal having a frequency which is obtained by dividing a frequency of an output signal derived from the VCO; a second frequency divider for dividing a frequency of a reference signal; a phase comparator for comparing a phase of an output signal of the first frequency divider with a phase of an output signal of the second frequency divider to output a phase difference thereof; and a charge pump for outputting the output signal of the phase comparator via a loop filter to a control voltage terminal of the VCO,
the frequency synthesizer is comprised of: frequency adjusting means for detecting a frequency error between both the output signal of the first frequency divider and the output signal of the second frequency divider, and for switching the capacitor value of the VCO, or the inductor value thereof in accordance with the detection result of the frequency error; and bias control means for applying an arbitrary voltage to the control voltage terminal of the VCO in order that the output signal of the charge pump is brought into a high impedance state when the frequency adjusting means is operated. With employment of such an arrangement, even when there is a manufacturing fluctuation occurred in the electronic components which constitute the VCO, since the resonant frequency of the parallel resonant circuit is varied in response to the actual oscillation frequency of the VCO, the phase can be locked at a desirable frequency. Furthermore, since the VCO can be manufactured in the IC form, the VCO can be made compact as well as in low cost.
Also, the frequency synthesizer is featured by that the VCO is equipped with a plurality of means for switching capacitors, or inductors. With employment of this arrangement, since the resonant frequencies are switched in a fine mode, the control sensitivity of the VCO can be lowered, and thus, the C/N characteristic can be improved.
Further, the frequency synthesizer is featured by comprising: first and second counters for counting the output signal of the first frequency divider and the output signal of the second frequency divider as a clock; time difference detecting means for detecting a time difference when the first counter produces a count end signal and the second counter produces a count end signal by employing a signal generated from the output signal of the VCO; and VCO control data producing means for producing a signal used to switch the capacitor value of the VCO, or the inductor value thereof in response to the output signal of the time difference detecting means. With employment of such an arrangement, since the frequency of the output signal of the first frequency divider is compared with the frequency of the output signal of the second frequency divider to switch the resonant circuits of the VCO, the phase can be locked at a desirable frequency even in such a case that the first frequency divider is such a frequency divider employed in the fractional N type frequency synthesizer in which the phase of the output signal is instantaneously changed.
Then, the frequency synthesizer is featured by comprising: first and second counters for counting the output signal of the first frequency divider and the output signal of the second frequency divider as a clock; time difference detecting means for detecting a time difference when the first counter produces a count end signal and the second counter produces a count end signal by employing a signal generated from the output signal of the reference signal source; and VCO control data producing means for producing a signal used to switch the capacitor value of the VCO, or the inductor value thereof in response to the output signal of the time difference detecting means. With employment of such an arrangement, since the signal used to detect the time difference is constant irrespective of the oscillation frequency of the VCO, the precision in detecting of the time difference can be continuously kept constant.
Also, the frequency synthesizer is featured by comprising: a time difference judging means for resetting both the first counter and the second counter in response to the detection signal of the time difference detecting means, and for applying an arbitrary voltage to the control voltage terminal of the VCO so as to bring the output signal of the charge pump into the high impedance state in the case that the time difference detected by the time difference detecting means becomes a value defined within a predetermined time difference. With employment of such an arrangement, since the PLL is set to the closed loop after the frequency synthesizer confirms that the oscillation frequency of the VCO is approached to the desirable oscillation frequency, this frequency synthesizer can be phase-locked at the desirable oscillation frequency by using the optimum VCO control data.
Further, the frequency synthesizer is featured by comprising: a loop filter control means for outputting a signal causing a time constant of the loop filter to be varied in response to the output signal of the VCO control data producing means. With employment of such an arrangement, even when the control sensitivity of the VCO is varied in response to the VCO control data, since the frequency response characteristic of the PLL is corrected based upon the time constant of the loop filter, the stable C/N characteristic can be obtained irrespective of the VCO control data.
Then, the frequency synthesizer is featured by comprising: charge pump control means for outputting a signal used to vary a current capability of the charge pump in response to the output signal of the VCO control data producing means. With employment of such an arrangement, even when the control sensitivity of the VCO is varied in response to the VCO control data, since the frequency response characteristic of the PLL is corrected based upon the current capability of the charge pump, the stable C/N characteristic can be obtained irrespective of the VCO control data.
Also, the frequency synthesizer is featured by comprising: a reset signal generation means for outputting a signal which is synchronized with the output signal of the reference signal source to both a reset terminal of the first frequency divider and a reset terminal of the second frequency divider when the operation of the frequency adjusting means is commenced. With employment of such an arrangement, since the starting time instant of the frequency dividing operation by the first frequency divider can be made coincident with the starting time instant of the frequency dividing operation by the second frequency divider, the detection precision of the time difference detecting means can be furthermore improved.
Then, the frequency synthesizer is featured by comprising: a first delay means for delaying the clock signal of the first counter, or the clock signal of the second counter. With employment of such an arrangement, since the starting time instant of the frequency dividing operation by the first frequency divider can be made coincident with the starting time instant of the frequency dividing operation by the second frequency divider, the detection precision of the time difference detecting means can be furthermore improved.
Then, the frequency synthesizer is characterized by comprising: a second delay means for delaying the output signal of the reset signal generation means to output the delayed signal to both the reset terminal of the first frequency divider and the reset terminal of the second frequency divider. With employment of such an arrangement, since the time duration defined after the resonant circuit of the VCO has been switched until the frequency becomes stable can be secured, the frequency can be adjusted in higher precision.
Furthermore, according to the present invention, this frequency synthesizer is provided in a wireless mobile appliance. With employment of such an arrangement, it is possible to provide a compact wireless mobile appliance manufactured in low cost, while improving a communication quality thereof.
Then, according to the present invention, this frequency adjusting means 9 is provided in a wireless base station apparatus. With employment of such an arrangement, it is possible to provide a compact wireless base station apparatus made in low cost, while improving a communication quality thereof.